Fine balls having diameters of about 1 mm or less, high sphericity, and an extremely sharp diameter distribution, such as bearing balls, solder balls for connecting IC packages, etc., are precisely classified into the predetermined diameter ranges. To obtain fine balls in the targeted diameter range, it is necessary to conduct a classification for removing fine balls having larger diameters than the upper limit, and a classification for removing fine balls having smaller diameters than the lower limit.
In the classification for removing fine balls having larger diameters than the upper limit, the fine balls that have passed through sieve holes (hereinafter referred to as “passing-through balls”) are determined as passed products, and those that have not passed through sieve holes (hereinafter referred to as “residual balls”) are determined as failed products. On the other hand, in the classification for removing fine balls having smaller diameters than the lower limit, the residual balls are determined as passed products, and the passing-through balls are determined as failed products.
Conventionally used as means for classifying fine balls having high sphericity and an extremely sharp diameter distribution are sonic sieves using electroformed flat sieves having holes with precisely controlled inner diameters, which are produced by electroforming methods (JP 2002-505954 A). Such a sonic sieve is generally a flat sieve having holes, on which fine balls are vibrated by sound waves so that they fall through the holes efficiently. In the classification for removing fine balls having larger diameters than the upper limit, only small numbers of failed products remain as residual balls on the sieve, while almost all fine balls pass through the holes. Accordingly, the classification is easy even with such sonic sieves.
However, in the classification for removing fine balls having smaller diameters than the lower limit, there is a problem that because there are a large percentage of the residual balls, they clog the holes of the sonic sieve. As a result, there is a high probability that the failed products, which should be passing-through balls, are mixed into the residual balls and thus determined as the passed products. Therefore, in the classification of the residual balls as the passed products by the sonic sieve using an electroformed flat sieve, it is necessary that the number of fine balls supplied onto the electroformed sieve should be reduced, and that a classification operation should be carried out for a long period of time.
However, the classification for a long period of time (long residual time of fine balls) leads to damage on the fine balls and the electroformed sieve. This problem is serious particularly when the fine balls are continuously supplied for high efficiency.
Also known is a roller classification machine for carrying out the classification of fine balls by rolling the fine balls between two rollers with a precisely controlled gap. However, in the case of the roller classification machine, only one layer of fine balls can be supplied between the rollers, resulting in low classification capacity and thus unsuitable for mass classification.